Recently, paper machines have been increasingly developed and improved, and especially, there is an obvious trend to increase speed and width of paper machines for enhanced productivity.
As for the wire part of paper machines, Fourdrinier formers have been replaced by on-top twin wire formers, and then gap formers to improve the drainage capacity. In gap former type paper machines, a stock jet delivered from the headbox is immediately sandwiched between two wire cloths so that the surface of the stock jet is less disturbed, resulting in good surface smoothness. Another advantage of gap former type paper machines is drainage from both sides of paper layers, which makes easier to control drainage levels so that they can operate at higher speed than Fourdrinier or on-top formers and the resulting paper shows little difference in surface smoothness between both sides.
In gap former type paper machines, however, sudden drainage from both sides of paper layers still at very low stock consistency causes the distribution of fines and filler in paper layers to be localized at surfaces and the amount of fines in middle layers of paper tends to decrease. For this reason, gap former type paper machines had disadvantages such as low internal bond strength and low stock and ash retention on the wire during the papermaking process.
Thus, coated printing papers using coating base papers prepared by gap former type paper machines have low internal bond strength so that even if water contained in the coated papers evaporates during heat drying after offset printing, the water cannot pass through coating layers, resulting in separation between paper layers and formation of blisters, i.e. pockets of coating layers, which may cause serious quality problems such as roughened printing surface. This limited the use of gap former type paper machines to the preparation of newsprints or the like.
In order to improve blisters in coated printing papers, the internal bond strength of coating base papers used should be increased. Generally, a method used to improve internal bond strength is to add a dry paper strength agent such as cationized starch or polyacrylamide during the papermaking process. However, even if a dry paper strength agent is added into a stock, it is more likely to be fixed to fines so that it must be added in large quantity to obtain sufficient internal bond strength when fines are localized, which causes problems such as poor freeness or formation. Especially, expensive polyacrylamide increases costs and affects formation due to high cohesion, thereby inviting print quality loss. On the other hand, cationized starch must be added in large quantity as compared with polyacrylamide, which may affect freeness, thereby inviting problems such as drainage failure, an increase in dry load, a decrease in wet web strength, etc.
A method for further improving internal bond strength by applying an external dry paper strength agent in addition to the incorporation of an internal dry paper strength agent has also been proposed (see JPA H10-280296). However, any dry paper strength agent cannot penetrate into base papers and sufficiently perform when fines are localized on paper surfaces as observed in papers prepared by gap former type paper machines, as described above.
Recently, various hardware improvements have been made to solve this problem. Conventional systems entailed significant localization of fines or ash on paper surfaces due to sudden drainage via an instrument such as a forming shoe, forming board, suction box or the like during the initial drainage step, but current so-called roll and blade gap former type paper machines allow for slow drainage by combining initial drainage via a forming roll having a suction with a drainage blade immediately downstream of it, and they also allow for even distribution of fines and filler in paper layers and good formation by applying microturbulence to wet web layers with the aid of a pulse force from the pressing drainage blade to promote the dispersion of fibers. Thus, extremely weak parts disappeared in paper layers, and dry paper strength agents added to the stock can effectively increase paper strength, thereby improving internal bond strength.
However, roll and blade gap former type paper machines improved paper layer structures by slowing initial drainage, but have not significantly improved stock retention loss, which is a problem with conventional gap former type paper machines, because fines and filler within wet web are expelled by pulses applied within wet web under the pressure of the drainage blade.
Thus, a technique for improving retention was proposed, comprising adding a cationic polyacrylamide, then adding an anionic inorganic microparticle such as bentonite or colloidal silica, and further adding an anionic polymer as retention aids to achieve high retention of fines while maintaining good formation (see WO2001/34910). However, sufficient improvement has not been achieved yet in internal bond strength, retention and formation under the current circumstances where the speed, ash content and DIP content are increasing.
On the other hand, on-machine coaters capable of in-line papermaking and coating have been widely adopted in recent years. On-machine coaters have the advantages over off-machine coaters that they are capital- and space-saving and enable rapid coating of base papers, thereby reducing production costs. However, papermaking and coating take place continuously so that a web break results in a significant production efficiency loss such as prolonged feeding period. Especially when a base paper is coated via an on-machine coater having a film transfer coater such as a metering size press coater or gate roll coater, and further coated via an in-line continuous blade coater, web breaks may be likely to occur by the presence of foreign matter on the surface of the base paper. Thus, foreign matter must be minimized for efficient operation of the blade coater, which limited the incorporation of deinked pulp and the like containing much foreign matter. In addition, paper strength must be enhanced to reduce web breaks, which limited the use of gap former type paper machines incapable of conferring high strength as described above.
Sources of the foreign matter include, among others, white pitch derived from coating layers contained in raw materials from defibered broke generated during coating (coated broke), stickies derived from deinked pulp, and natural pitch derived from mechanical pulp. A known measure against such foreign matter including white pitch, stickies and natural pitch is to add a cationic polymer called coagulant to coated broke raw material, deinked pulp or mechanical pulp before mixing during the stock preparation step (JPA 2005-206978, JPA 2005-179831, JPA 2005-133238, JPA 2004-60084, JPA 2001-262487, Japanese Patent No. 3681655, JPA 2005-2523). Generally, coagulants are thought to neutralize the surface charge on anionic colloidal particles including white pitch, stickies and natural pitch so that the anionic colloidal particles are loosely fixed in the form of smallest possible particles to fibers to form soft flocks, thereby reducing problems of foreign matter.
Various methods for adding a coagulant to a raw material before mixing have been reported. For example, they include adding a coagulant to waste paper pulp before it is fed to the raw material preparation step of a paper machine (JPA 2005-206978), adding a coagulant to waste paper pulp before it is fed from the waste paper regenerating step to the mixing chest (JPA 2005-179831, JPA 2005-133238), adding a coagulant to a plurality of stocks during the stock preparation step before they are fed to the headbox (JPA 2004-60084), adding a cationic water-soluble polymer to a raw material based on magazine waste paper before mixing (JPA 2001-262487), etc. Other methods have also been reported, including adding a cationic water-soluble polymer to each of one or more papermaking raw materials before mixing and then adding a cationic polymer retention aid to a raw material mixture containing the papermaking raw material mixed with other papermaking raw materials (Japanese Patent No. 3681655), adding a cationic polymer during the defibering step after a mixture of recovered clarified water and coated broke has been combined with another pulp (JPA 2005-2523), etc.
However, coagulants have the disadvantages that the effect of the coagulants added to raw materials gradually decrease through steps and fixed colloidal particles are detached especially in high-speed paper machines generating a strong shearing force, because the coagulants form soft flocks loosely bound to fibers as described above. This required excessive amounts of coagulants to be added to neutralize the charge of colloidal particles again or additional amounts of retention aids to be incorporated to fix detached particles again, which invited not only a cost disadvantage but also problems such as secondary deposits formed by foreign matter modestly grown into coarse particles and excessive amounts of cationic chemicals. Generally, it is known that when a cationic chemical having a high molecular weight is added to coarse particles of foreign matter, the coarse particles of foreign matter are fixed to paper, resulting in an increase of paper defects or web breaks.
Another known method is to add a mixture of a cationic polymer and a cationic monomer to a papermaking raw material composition containing a plurality of pulps (JPA 2003-183995). However, this method comprises adding the coagulant after colloidal substances have grown into coarse particles or foreign matter has been destabilized upon contact with other pulps or chemicals, which may cause problems of foreign matter on paper surfaces and rather lead to web breaks.
Still another report proposes a method comprising adding a cationic retention/freeness aid in a papermaking system wherein at least one of a polyvalent metal salt and a cationic polymer is divided and added to at least two sites (JPA 2000-282390). In this method, however, the cationic polymer is added to a stock containing raw materials in order to improve retention, which rather positively encourages colloidal substances or the like to form coarse particles. Thus, this method cannot reduce runnability problems such as deposits from coated broke, deinked pulp and mechanical pulp or web breaks as described above, but rather may induce these problems.
Still another report proposes to add a coagulant during the step of preparing a stock containing a plurality of pulps and the step of feeding it from the headbox to the wire part (JPA 2006-138044). This method comprises adding the coagulant upstream of the screen downstream of the secondary pump to the stock containing a lot of white water typically to a solids content of less than 1.5% downstream of the headbox, and further adding a flocculant downstream of the screen. However, this method also fails to reduce runnability problems such as deposits from coated broke, deinked pulp and mechanical pulp or web breaks as described above, but rather may induce these problems.
In this manner, conventional techniques could not avoid problems such as deposits from coarse particles of colloidal substances or foreign matter and could not sufficiently overcome productivity loss, especially during the preparation of coating base papers in high-speed paper machines. To fix this foreign matter to fibers, excessive retention aids had to be added, resulting in paper quality loss such as uneven formation or filler distribution. Especially when a coated paper is produced continuously in-line using a coater from a coating base paper prepared in a high-speed paper machine such as gap former type paper machine, runnability problems such as web breaks could not be avoided, resulting in productivity loss and sometimes paper quality loss.